How to insert new row without duplicating existing data

I want to insert rows in my table like so: my columns are student,subject,class,teacher,level. Primary key is (student,subject). The table contains all the students, but the Math subject is missing for some of them, so I want to add it without duplicating the ones that already have it.

I’ve tried this but it gives me unique constraint violated:

insert into table (student,subject,class,teacher,level)
select a.student, 'math', null, null, null
from   table  a
where  a.student in (select distinct student from table where subject not in 'math') 
and    (a.student,a.subject) not in (select student,subject from table);

Answer

I think you basically need select distinct:

insert into table (student, subject) 
    select distinct a.student, 'math'
    from table a
    where not exists (select 1
                      from table a2
                      where a2.student = a.student and
                            a2.subject = 'math'
                     );